Various advances in commercial wireless and networking technologies have enabled the support of voice and high-speed data services to wireless unit end users, e.g., those using mobile phones, wireless personal digital assistants (PDA's), or the like. As third generation wireless packet data networks have evolved to support a wide range of multimedia and other high-speed data services, packet data voice services have become a viable alternative or replacement for traditional circuit switched voice communications. Because of the increased demand for wireless communications generally, and especially in terms of high-speed data transfer, service providers have sought to increase network capacity. However, supporting the need for increased capacity can negatively affect quality of service, particularly if no special attempt is made to address this need. For example, signal distortion resulting from co-channel interference may increase as channel load increases.
In a simple wireless system, a transceiver for receiving and transmitting RF signals is provided with a single transmit/receive antenna element, and possibly a second antenna element to provide reception diversity. To improve quality and/or increase capacity in a wireless network, more complex antenna systems may be used instead, e.g., an antenna array. An antenna array is a group of spatially distributed antennas/RF sensors, wherein the output of the antenna array is obtained by properly combining each antenna output by way of a weighting network, a beamforming network, or the like. An antenna array can reduce signal interference, increase effective received signal energy, and/or boost the signal-to-interference-plus-noise ratio (“SINR”) according to the signal arrival angles and/or directions of arrival. One type of antenna array is the adaptive antenna array or so-called “smart antenna.” An adaptive antenna array is an antenna array that continuously adjusts its own pattern by means of feedback control. Typical adaptive antenna arrays have the same architecture for the forward link and reverse link channels. With a limitation on the number of RF feed cables per base station, this results in a maximum of four co-polarized columns, two dual-polarized columns, or three columns of one polarization and one column of the other polarization. This leads to a suboptimal performance in suburban or light urban environments across the reverse link and/or the forward link, depending on the configuration choice.
To explain further, in modern wireless systems one of the most severe limitations imposed on adaptive antenna arrays is the number of RF feed cables per base station. Since tower top transceiver electronics are uncommon, antennas are typically connected to the base station electronics (e.g., housed at ground level in a building or cabinet) by way of large-diameter, low-loss RF feed cables. The cables impose significant weight and wind loading on the base station tower, and their maximum number is typically restricted to 4 per sector or 12 per cell. (A cell is a geographic area served by a base station, and a sector is a portion or subsection of that geographic area, e.g., a 60° or 120° “slice” of a cell.) Base station architectures are typically designed accordingly, to support a maximum of 12 cables per cell. Under this limitation, current adaptive antenna arrays typically have one of the following three architectures: a 4-column, single polarization array; a 2-column, dual polarization array; or a 4-column array with three columns at one polarization and a fourth column of orthogonal polarization. Antenna configuration remains the same for both uplink and downlink.
Each of the three array configurations has certain disadvantages in suburban or light urban environments. A 4-column, single polarization array provides the highest possible gain over the forward link (e.g., the RF channel for transmissions from base station to wireless unit), with either fixed beamforming or a per-user steered beam solution. On the reverse link, however, due to the high degree of correlation between signals received at four closely spaced antennas, only aperture gain is available. Diversity reception gain is very small or nonexistent, possibly resulting in a significantly smaller gain over the reverse link when compared to a non-adaptive antenna configuration of the typical cell site, which may have a single antenna column for transmissions and a pair of diverse antenna columns for reception. A 2-column, dual polarization array provides high gain, including both diversity (polarization) and aperture gain over the reverse link, due to uncorrelated signals received at two orthogonally polarized pairs of antennas. Over the forward link, a combination of beamforming and transmission diversity is used, however, which may result in gain values lower than in a 4-column, single polarization array as other forms of diversity exist in modern cellular systems. A 4-column, “3/1” polarization array is a compromise, providing performance similar to a 2-column, dual polarization array.